LONDON (REUTERS).- London's National Gallery said on Wednesday it would crack down on the re-sale of tickets to its blockbuster show of Leonardo da Vinci paintings which are being offered online at up to 300 pounds each ($470).

The normal price of a ticket to "Leonardo da Vinci: Painter at the Court of Milan," billed as the most complete collection of Leonardo's few surviving paintings ever staged, is 16 pounds $25).

"We are obviously very disappointed at the resale of these tickets for profit," said a spokeswoman for the gallery.

"The resale of tickets for the Leonardo da Vinci exhibition is against the terms and conditions of their sale and this information is printed on the tickets.

"Our website clearly states: 'Tickets that have been resold will be canceled without refund and admission will be refused to the bearer.'"

She added that the gallery was in the process of contacting companies and websites offering to re-sell tickets and requesting that they "stop immediately.

"We are conducting checks on Leonardo tickets."

The exhibition, which opened on November 9 to rave reviews and closes on February 5, 2012, quickly sold out as art lovers from Britain and abroad scrambled for access.

The gallery has withheld a limited number of tickets to be sold each day, but on its website it warns visitors they may have to wait for up to three hours and then wait again to enter the show.

The website also advises ticket holders unable to attend to provide a signed letter of authorization if they wish to hand them over to somebody else.

On Wednesday, two tickets were offered on the eBay online auction website for 600 pounds ($940), although most were priced significantly lower and in some cases at a discount to face value.

On the Viagogo online ticket exchange a pair of tickets was on sale for up to 227 pounds ($350).

The exhibition boasts nine of 15 or 16 known Leonardo paintings, including "Christ as Salvator Mundi" which was only recently attributed to Leonardo, although its authenticity is still questioned by some.

Listed by the National Gallery as an original, the painting was sold at Sotheby's for 45 pounds in 1958, when it was believed to be by one of Leonardo's pupils.

According to ARTnews, the work is now owned by a consortium of dealers, including Robert Simon, a specialist in Old Masters in New York. Valued by experts at up to $200 million, Simon told the publication the work was not for sale.

The National Gallery has collected virtually all of the known Leonardo paintings from Milan, where he was court artist to the city's ruler Ludovico Sforza from around 1482-1499.

(Reporting by Mike Collett-White, editing by Paul Casciato)